The present application is the U.S. national stage application of International Application PCT/NO01/00003, filed Jan. 3, 2001, which international application was published on Jul. 12, 2001 as International Publication WO 01/49392. The International Application claims priority of Norwegian Patent Application 20000088, filed Jan. 7, 2000.
The invention relates to a bag filter device, which allows collection and draining of particles of liquid from a gas flowing through the bag filter. Such a bag filter is used to separate preferably solid particles, e.g. dust, sand or pollen from a gas flow, the bag filter being used, for example, in a ventilation plant, combustion engine or in connection with industrial filtration of gasses.
A bag filter is normally formed of several suitably joined filter pockets formed of a suitable filter material. The joined filter pockets are normally secured to a common frame and thereby form a usable filter. The number of filter pockets in the filter is normally decided on the basis of the area of use of the filter and of the flow rate of the gasses through the filter.
Otherwise the invention relates to the problems which may arise as a consequence of the filter absorbing and being wetted with liquid from a gas flowing through the filter.
Bag filters for separating preferably solid particles from a gas flow, are used either alone or in combination with other filters or devices, for example a droplet separator or a cyclone separator. Such a bag filter may be formed of fibre glass material or synthetic material, the filter material being woven or non-woven. For air purification bag filters of non-woven synthetic material are preferred.
A bag filter may consist of one or more materials. The bag filter may be constructed in layers, each layer being formed of a homogeneous material of distinct properties, the desired properties of the bag filter being made up of the properties of the layer materials together. Alternatively, the bag filter may have a progressive structure, in which the properties of the bag filter change gradually and suitably through the filter wall, for example in that the filter material is formed with gradually decreasing pore sizes in the downstream direction of a gas flow, so that the bag filter becomes gradually tighter and can trap particles of gradually smaller sizes.
In air purification such a bag filter is normally positioned in or near an air inlet, the bag filter in many cases being placed immediately after a device, for example a droplet separator, which is to separate water which is drawn into the air inlet together with the air. Such water is formed from rain water, mist or sea spray, for example. In an arrangement consisting of a droplet separator and a downstream bag filter, most of the water will be separated from the air in the droplet separator, whereas a minor portion of the water will follow the air flow further into the bag filter. Without a droplet separator the bag filter will receive a comparatively larger amount of water than in that case where a droplet separator is used under corresponding conditions. In both cases the bag filter receives air, which contains water particles in the form of droplets of varying droplet sizes. In a manner corresponding to that in which the bag filter traps solid particles of a certain size, it could trap water droplets of a certain size. This causes the bag filter to be wetted by water, possibly another liquid which might be present in the gas flow, and the whole or parts of the bag filter possibly to become saturated with water and/or other liquid present.
In heavy fog, for example, the atmospheric air will contain large amounts of water in the form of minute water droplets of a size of 30 micrometers or smaller, and these water droplets are separated only partially in a droplet separator. Thereby a large amount of the droplets are carried further into a downstream bag filter, in which bag filter a large amount of the droplets of a size above 1 micrometer are adsorbed or stopped, and in which the degree of wetting depends on the filter type and its configuration. Water trapped in the bag filter may gradually permeate the bag filter and exit on the downstream clean side of the bag filter, after which some of the water in the form of water droplets may be caught by the air flow and carried further therein. A common method of preventing such water droplets in an air flow from being carried further into, for example, a ventilation plant in a building, is to position a droplet separator immediately downstream of the bag filter, in which case two droplet separators and an intermediate bag filter are used.
Bag filters for use in gas flows other than air flows, possibly in combination with one or more droplet separators and/or other equipment for treating a flowing gas, may be wetted in a corresponding manner by liquids present in the gas flow, after which the liquid or liquids may possibly permeate the bag filter and be carried further downstream in the gas flow.
There are several drawbacks to such bag filters, and where such a bag filter is possibly used alone or in combination with, for example, one or more droplet separators and/or other equipment for treating a flowing gas.
As a consequence of gravity, water or liquid particles adsorbed or trapped in a sufficient amount in such a bag filter, will be pulled down towards a lower filter portion and finally through a bottom portion of the bag filter. The liquid could then drip out downstream from the clean side of the bag filter and possibly be caught again by the flowing gas. Additionally, the flowing gas could possibly contain droplets of liquid of such a small size that they will not be trapped in the bag filter. Relatively large drops dripping out from the bag filter may be caught and drained out in a possible downstream droplet separator. However, many of these droplets and the droplets passing unobstructed through the bag filter are very small, normally of a size smaller than 30 micrometers. These droplets are caught only in part by a downstream droplet separator, whereas the remaining droplets are carried further in the gas flow.
In a ventilation plant, for example, this may have undesired effects, as water droplets that permeate the bag filter are often saturated with salt, possibly they contain bacteria or other contaminants, and these substances are thereby carried further in the ventilation plant, possibly affecting adversely the surroundings that the air makes contact with. Such contaminants may possibly settle at one or more points in the ventilation plant, for example in a downstream droplet separator or in a drip zone below or round the bag filter. By possible subsequent drying, such deposits may be torn loose and carried further by the air flow. Alternatively, such deposits may in time accumulate and possibly cause equipment and parts of the ventilation system located downstream of the bag filter to become blocked or work less efficiently.
Water droplets permeating the lower portion and bottom portion of a bag filter and dripping out of the bag filter, may often be too big to be carried along by the air flow through the bag filter, or the water droplets may be in an area around the bag filter with less air flow, so that the water droplets drip down onto a bed and are possibly drained away. Lack of draining possibilities, possibly insufficient draining, may lead to damage by damp or rot in the ventilation plant or otherwise in the surroundings.
Droplets of liquid dripping out from a bag filter are often removed in a droplet separator or similar device placed downstream and in the proximity of said bag filter. In those cases where such an arrangement is necessary in order to bring about a desired treatment of a flowing gas, the installation costs increase, as such an arrangement demands more equipment and space than a plant in which the bag filter constitutes the last stage in the gas treating process. Additionally, a droplet separator or similar device located downstream of said bag filter will make the overall pressure fall in the gas flow of the plant increase, which increases the continuous operating costs of the plant compared to a plant in which the bag filter constitutes the last step in the gas treating process.
The object of the present invention is to provide a bag filter with a device which allows the collection and draining of particles of liquid from a gas flowing through the bag filter, so that the liquid particles are prevented from permeating and dripping out from the downstream clean side of the bag filter. Thereby the above-mentioned drawbacks are prevented or reduced.
As appears from the characterising part of the present independent claim, the object is realised in that, in a portion of the bag filter as a whole or in an outer filter layer, the filter material is formed of a liquid-tight material. When the bag filter is in use, this portion is formed by a lower portion which has, in its continuation, a bottom portion of the same bag filter arranged thereto, so that, due to gravity, separated liquid from a gas flowing through the bag filter will finally accumulate in this bottom portion, and may possibly be drained through the bottom portion. The bottom portion of the bag filter is possibly formed or provided with a liquid outlet, preferably at a lowermost position. Liquid accumulated in the bag filter may for example be drained through a drain pipe or a similar draining device. Alternatively, the liquid may be drained through the bottom portion and out through the open gas inlet of the bag filter, the liquid running counter to the flow of the gas through the bag filter due to gravity, and the liquid being in an area round the bag filter with normally little gas flow. The bag filter is preferably formed from synthetic and non-woven material.
The collection of, for example, water in the bag filter, may be made more efficient in that the entire filter material, or parts thereof, is/are made water-repellent, terminologically called hydrophobic. This may be achieved by means of suitable chemical treatment of the filter material, the pore walls of the filter material in this way being impregnated and made hydrophobic. This causes surface tensions between water particles and the pore walls of the filter material to be reduced, water particles to adhere to the pore walls to a smaller degree and thereby separate more easily from the filter material, more water particles to unite more easily with larger water drops, larger water drops more easily being pulled downwards, as a consequence of gravity, gathering in the lower portion and bottom portion of the bag filter.
In a layered bag filter it will be the most suitable, in the collection of, for example, water from air flowing through a filter, that the last downstream filter layer has been made hydrophobic, whereas in a bag filter of progressive structure it will be the most suitable that a downstream filter portion has been made hydrophobic. Thereby most of the water can be adsorbed in the upstream filter layers or filter portions, whereas downstream filter layers or filter portions prevent water from permeating, the filter layers or filter portions, however, allowing at the same time air to pass through the bag filter.
When such liquid-repellent bag filters are being used on other gasses than air, and when the separated liquid, possibly liquids, are different from water, all or parts of the filter material must have been made liquid-repellent to the liquid or liquids that will contact such a bag filter. This may for example be achieved through suitable chemical treatment of the filter material, which will, in use, result in a more efficient separation of liquid in the bag filter.
The invention has the effect that the particles of liquid, which are present in a gas flow and are trapped and concentrated in the bag filter, may gather in the lower portion and bottom portion of the bag filter, without, at the same time, flowing through the bag filter and dripping out from the clean side of the bag filter, and that liquid can efficiently be drained from the bottom portion of the bag filter, for example through drain pipes or drain ducts, so that droplets of liquid do not cause the above-mentioned drawbacks in equipment and surroundings.